The natural environment of animals and human beings contains a large variety of infectious agents such as viruses, bacteria or fungi. Many of these infectious agents may cause diseases in the infected hosts. Under normal circumstances the infected host recovers from the disease induced by the infectious agent after a certain period of time. This recovery is due to the immune system of an animal, including a human being.
The immune system is the part of the human or animal body that is responsible for eliminating the infectious agent. The immune response is divided into a specific and an unspecific (innate) reaction, although both cooperate closely. The unspecific immune response is the immediate defense against a wide variety of foreign substances and infectious agents. In the innate immune response against viruses, Interferons (IFN)-α and IFN-β are absolutely essential to control initial virus replication and to activate natural killer (NK) cells for immediate destruction of infected cells. Intracellular bacterial or parasitic pathogens induce IL-12 that up regulates IFN-γ in NK cells and/or some T cell subsets. IFN-γ activated NK cells can kill intracellular pathogens. Moreover, IFN-γ also activates macrophages and enables them to kill internalized pathogens.
By far, the richest source of IFN-α/β on a per cell basis are dendritic cells (DC); a specialized cell population strategically distributed throughout the body. Plasmocytoid DC or CD11c+ CD8+ DC are among the best producers of IFN-α/β. CD8+ DC infected with intracellular non-viral pathogens are crucial cells able to secrete IL-12, which is essential in the early steps of immune defense.
A specific immune response can be induced against a particular foreign substance (antigen) after a lag phase, when the organism is challenged with this substance for the first time. Initiation of a specific immune response is also coordinated by DC. There is a constant traffic of these cells from the periphery to the secondary lymphoid organs, the lymph nodes, or spleen where naive T and B cells recirculate. Antigen that is carried by DC to these organs enables activation of naive T- and B cells to become effector T- and B cells. For this, DC not only carry the antigen, but in addition, the plasticity of pathogen recognition allows different gene activation in DC and thus, a pathogen adjusted priming of T cells.
The specific immune response is highly efficient and is responsible for the fact that an individual who recovers from a specific infection is protected against this specific infection. Thus, a second infection with the same, or a very similar, infectious agent causes much milder symptoms, or no symptoms at all, since there is already a “pre-existing specific immunity” to this agent. Such immunity and immunological memory persist for a long time; in some cases, even a lifetime. Accordingly, the induction of an immunological memory can be used for vaccination, i.e. to protect an individual against infection with a specific pathogen.
For vaccination, the immune system is challenged with a vaccine that is less harmful than the pathogenic agent against which an immune response is to be induced. The vaccine comprises or expresses epitopes that are found in, or expressed by, the agent against which the vaccination is done. The organism, thus, is immunized against the agent containing the epitope that is part of the vaccine.
Typical vaccines are attenuated or inactivated viruses (e.g. the polio or small poxyirus vaccines), recombinant proteins (e.g. recombinant Hepatitis B virus S-protein), heat inactivated bacterial toxins (e.g., Clostridium tetani toxin), or polysaccharides of the bacterial capsule wall (e.g., Streptococcus pneumoniae).
Since infectious diseases may lead to critical conditions in newborns and sucklings, there is an interest in vaccinating children and/or newborn animals as early as possible. Examples of conditions against which a vaccination is desirable are poxyirus infections, including smallpox. Attempts to successfully vaccinate newborns, however, are hampered because the immune system of newborn mammals is not yet mature. The immune system of neonatal infants and mammalian animals is thought to mature gradually over a certain period of time. For humans this maturation occurs during the first year of life. This is the reason why the neonatal age group is left open to various infections during this first year (Gans et al., J. Am. Med. Assoc. (1998) 280, 527–532). More particularly, neonatal infants have impaired B-cell function, deficiencies in primary antigen presentation by dendritic cells, and limited T-cell proliferation (Gans et al., J. Am. Med. Assoc. (1998) 280, 527–532). Shortly after birth, the levels of T cells in the spleen are 1,000 fold lower than in adults. In order to achieve at least a weak immunization, it has been suggested to use either replicating viruses, or formulations comprising an adjuvant, for immunization. However, with replication viruses there is always a risk that the immature immune system may become overwhelmed by viral infection or live viral vaccines, since T cells are necessary for viral clearance (Hassett et al., J. Virol. (1997) 71,7881–7888). Since there is a reduced production of cytokines by Th-1 helper T cells in neonates, the response in infants is predominantly Th-2. Consequently, cytotoxic T cells are not recruited and viral clearance is not achieved.
The situation in mammalian animals is very similar to the situation in humans, i.e.
the immune system after birth is not yet mature. In newborn mice, the number of splenic CD4+ T cells and CD8+ T cells is respectively, 80,000-fold and 1,000-fold lower, than in spleen cells of adults. Moreover, the interferon (IFN) producing system is immature in these mice. Therefore, IFN in neonatal mice is unable to efficiently control the expansion of intracellular pathogens at the site of infection. In addition, the low number and possibly inadequate activation stage of immune cells are too limited to cope with the rapidly expanding pathogens or replicating viruses used for vaccination.
Due to the risk associated with live viral vaccines, it is not recommended to vaccinate neonatal animals, including humans, with replicating viruses. For example, it is not recommended to vaccinate newborns against smallpox with the vaccinia virus strains that had been used up until the eradication of smallpox, i.e., strains such as Elstree, Copenhagen and NYCBH. According to recent recommendations in the USA, babies younger than 12 months of age should not receive the smallpox vaccines commercialized thus far.
The vaccination of neonates with formulations comprising an adjuvant has the disadvantage of introducing numerous harmful substances into the body. Thus, vaccination in human neonates is only done in emergency cases, e.g. in Hepatitis B virus infection.
In summary, it is to be noted that the immune system is not mature at birth. Since vaccination with replication competent viruses or formulations comprising an adjuvant have significant disadvantages; infants are not vaccinated before the age of 2 month in Germany (Empfehlung der Ständigen Impfkommission STICO, 2001) or 6 weeks in the USA (ACIP “Recommended Childhood Immunization Schedule, United States”).
The delay in the development of the immune system is compensated in part by the transfer of maternal antibodies from the mother to the suckling during pregnancy or breastfeeding. However, not all infants are breastfed due to various reasons. Thus, there is a very critical period of time of about 6–8 weeks in humans during which the infant, having an immature and thus not fully functional immune system, does not receive maternal antibodies, and during which vaccination is usually not successful, or too dangerous.
The situation is very similar in mammalian animals, in particular for economically important animals such as cows, or companion animals, such as cats and dogs. To reduce costs, the amount of milk the calf receives from the mother is often drastically reduced. Instead, the calf receives a mixture of milk powder, starter and specific concentrated feed, sometimes already in the first week after birth. Consequently, the calf does not receive the necessary amount and variety of maternal antibodies, so the immature immune system is very susceptible to infections. Furthermore, farmers who breed calves and those who raise them for meat production are often not the same. At 4 to 6 weeks of age, calves from different breeder farms are pooled and shipped to other farms for meat production. At this time, when maternal antibodies are low and the immune system is not fully developed, the animals are exposed to new infectious agents under stressful conditions. This increases the risk for infections that could be prevented by vaccination. A similar situation can be found in cat or dog breeding facilities where the risk of infection is high.